Garage conversion - building the floor

[akist]

We are converting the double garage to a small apartment.

For the floor it has been suggested we place 100mm of insulation (GA4000 Celotex) followed by 65mm reinforced concrete screed.

I understand its purpose is to provide heat insulation but the GA4000 is foam and am intuitively concerned about placing a slab of concrete on foam.

As I understand it this concrete slab is simply laid and floating over the celotex with no further connection to the structure. It sounds flimsy and precarious, for example I am thinking if I place something heavy, or just on its own, the whole slab might move enough to produce cracks on tiles or on various walls.

Doing some simple calcs, the structure is 50m2 and the thickness is 65mm so I work thjis out to be 3.25 cubic metres and a weight of 7,800 kg. This sounds very heavy but it is really 156kg/m2. Supposing I move my piano there, the piano is about 0.75m2 in area and weighs 235kg so I am placing 313kg/m2 over a surface that is 156kg/m2. I understand the concrete slab spreads this weight around.

Can someone please tell me if I am on the right track? Are there other floor construction techniques I could consider?

 

[Deleted member 174758]

It's actually not an uncommon method of floor insulation these days and I've seen it used in commercial (hotel) kitchens, public areas in office blocks, etc as well as in domestic builds. Providing the screed is sufficiently thick it is normally pretty robust - although if you want to put something like a piano on it you may need to look at alternatives such as insulating screed. If you do go the Cellotex route, remember to lay plastic sheeting on it (only needs to be thin) to form a secondary DPM between the insulation and the screed

 

[^woody^]

Can someone please tell me if I am on the right track?

You are not. It's common and acceptable.

 

[Leofric]

65mm screed on vapour barrier on 100mm insulation on 150mm concrete on Visqueen dpm on 50mm sand blinding on 150mm well consolidated and compacted hardcore is a normal ground floor construction. The Celotex is thermal insulation as required under building regs to comply with Part L along with insulation to external walls and roof ( as well as other things ). You require building regs approval for the whole building. There are various forms of floor construction as there are various forms of construction for walls and roofs or any part of a building. The floor is separated from the walls by perimeter insulation. I have heard of cases where a timber floor finish on top of insulation can appear a bit unstable but not aware of this with a concrete screed finish.